Is there a serial link difference for AT mode vs. API mode?

2 Xbee’s (V.2): Coordinator with End device direct connected (Dout) to Pololu Micro SSC with servos.
Everything works great in AT mode (ZNET 2.5)
However, changing firmware into API (ZNET 2.5 Coord and End) gives immediatly a “fatal error” on the SSC (Yellow LED on, Red blinking). This occurs on switching “power on”.
API-frames can be sent succesfully to End Xbee.
Even loopback test gives good results on remote.
But Pololu stays in alarm, at whatever baud rate, and does nothing.
Q.: The same hardware configuration, working in AT and not in API?

Here is the link which brief about API mode,

API mode is a Frame Based data structure that is specific to the devices you are using. In this case, the data packets from your Pololu Micro would have to implement Digi’s API frames as apposed to transparent mode where data in = data out.